This invention relates generally to optical-waveguide modulators and more particularly to an integral, interferometric, optical-waveguide, amplitude modulator which has a linear response to small applied voltages (approximately less than 100 volts).
Interferometric waveguide modulators, which are integrated optical analogues of Mach-Zehnder bulk interferometers, are commonly used to provide amplitude modulation in integrated optical circuits. In existing embodiments, the waveguide arms of the interferometer are of equal physical length, or balanced. When voltages are applied to electrodes near the waveguide arms, electric fields are formed across the waveguides and, through the electro-optic effect, the optical path lengths of the two arms are made to differ. This difference in optical path length results in mode conversion effects at the output branch which results in the desired amplitude modulation. However, the resulting amplitude modulation is quadratic in response to an applied voltage. Such a response is disadvantageous for many applications such as electromagnetic field measurement sensors and for optical polarization-independent operation, that is, amplitude modulation which is independent of the polarization state (TE or TM) of the optical mode. Such applications and a more practical use of interferometers require a linear response to applied voltages. For a linear response, a built-in phase bias or phase-shift of .pi./2 radians is required. The problem is how to provide the bias of .pi./2 radians. In bulk electro-optic modulators this is usually accomplished electrically, i.e., by applying a d.c. half-wave voltage, that is, a d.c. voltage for retarding optical energy by one-half of a wavelength, or by the insertion of a birefringent (quarter-wave) plate in the optical path. Bias voltages are undesirable for electromagnetic field measurement sensors because the voltages substantially decrease the versatility of the device. Bias voltages are difficult to apply for optical polarization-independent operations due to the complexity of the electrode configuration. The waveguide nature of interferometers does not allow the insertion of classical waveplates in the optical path.
Providing the bias of .pi./2 radians by applying thin film overlays on one of the waveguide arms also is difficult because of the small separation (.perspectiveto.11 microns) between the arms and the additional fabrication steps. Overlays may also require tuning to achieve a proper change in phase.